There is known a mobile wireless communication system that makes wireless communication possible using radio channels such as a control channel, a physical communication channel, and a direction communication channel. Here, the control channel is a radio channel used between a base station and mobile stations in order to perform a call and reception of a call in the mobile stations and other kinds of communication control. The physical communication channel is a radio channel used between the base station and the mobile stations such that each of the mobile stations performs a call and data communication with the other mobile stations and stationary terminals (e.g., a stationary telephone and a network-connected computer terminal) via a mobile communication network. The direct communication channel is a radio channel used such that each of the mobile stations directly performs a call and data communication with the other mobile stations without the intervention of the base station.
In digital communication using such radio channels, transmission data may be transmitted as a predetermined frame structure (e.g., Non-Patent Document 1). FIG. 9 shows frame formats of a control channel (a physical control channel), a physical communication channel, and a direct communication channel defined in p. 115 to 116 in the second issue of Association of Radio Industries and Businesses “Narrow Band Digital Communication System (SCPC/FDMA) Standard ARIB STD-T61, ver. 1.0, second issue” (May 27, 1999, p. 115 to 117, p. 171 to 174) as ARIB STD-T61 that is the standard of a narrow band digital radio system.
Here, sync words used for frame synchronization often have an identical length in all of the control channel, the physical communication channel, and the direct communication channel. In an example shown in FIG. 9, all sync words are fixed to 20 bits. Sync words of a fixed length are defined in ARIB STD-27 that is the standard of a PDC (Personal Digital Cellular) as a mobile telephone system. It is possible to share detection of frame synchronization in digital signal processing among the channels and simplify processing by standardizing lengths of the sync words as an identical length. For example, when the control channel is shifted to the physical communication channel or when the direct communication channel is started, in order to improve synchronization performance, a special frame for synchronization (a synchronization burst) is used. In this case, sync words longer than usual (i.e., having 32 bits) are used.
As a method of capturing sync words in a communication apparatus on a reception side, various methods are known. For example, a large number of methods of capturing sync words by specifying a correlation value between a sync words known on the reception side and a baseband signal waveform or the like demodulated from a reception signal according to a predetermined arithmetic processing are used. In the standard of ARIB STD-T61 described above, it is judged that, with one frame as 40 msec (milliseconds), synchronization is established when a sync word of 20 bits is successfully received twice. Therefore, sync words for 40 bits are used in a period of 80 msec until synchronization is established.
In a micro-cell system such as the PDC, the control channel and the physical communication channel are not always provided by an identical base station. Therefore, it is technically difficult to always maintain timing between the control channel and the physical communication channel constant. Because of such a fact, the standard described above does not specifically define frame timing between the control channel and the physical communication channel.
When wireless communication is performed using the physical communication channel and the direct communication channel, it is required to quickly establish a communication link between a transmission side and a reception side. When a delay occurs in establishment of the communication link, for example, a problem occurs in that the beginning in a call is cut. In addition, when digital sound data is transmitted and received, an information amount increases to an enormous amount. An information amount that should be transmitted in the physical communication channel and the direct communication channel may increase compared with that in the control channel. As a modulation system having satisfactory transmission efficiency, a QPSK (Quadrature Phase Shift Keying) modulation system is known. The QPSK modulation system is adopted in the PDC and a mobile wireless communication system conforming to the standard of ARIB STD-T61. However, it is also known that, in the QPSK modulation system, it is difficult to cover a wide area with one base station.
In a mobile wireless communication system in which data communication is performed, widening of a band of an occupied frequency is in progress in order to improve communication speed (rate). On the other hand, in a radio apparatus mainly used for voice call, narrowing of a band of a radio channel is in progress in order to improve efficiency of use of a frequency. In the radio channel having a narrowed band, information transmission speed (capacity) inevitably falls. Therefore, the length (the number of bits) of a sync word is preferably smaller. However, when the sync word is shortened, performance of frame synchronization is deteriorated. In particular, when there are a lot of noise on the radio channel, there is a problem in that a rate of normally establishing synchronization falls to cause an increase in mis-synchronization.
In a mobile wireless communication system set in a suburb or the like in the United States, in order to make it possible to cover a wide area with one base station, a power amplifier with high power may be used at a transmission output stage in a base station. In a modulation system (e.g., QAM: Quadruple Amplitude Modulation) in which an envelope of a modulated wave signal is nonlinear, it is necessary to perform amplification using only a part where linearity can be maintained in an input/output characteristic of the power amplifier and power efficiency in the power amplifier falls. Therefore, it is preferable to use modulation systems such as the FM (Frequency Modulation) and the PM (Phase Modulation) in which an envelope of a modulated wave signal is substantially linear. However, in the FSK (Frequency Shift Keying) modulation system, transmission efficiency is about half compared with that in the QPSK modulation system and information transfer speed (capacity) falls. In this case, as in the above case, the length of a sync word is preferably small. However, when the sync word is shortened, the problem described above occurs.
The invention has been devised in view of the above-mentioned circumstances and it is an object of the invention to provide a mobile wireless communication apparatus and the like that makes it possible to establish a communication link by reducing the length of a sync work to reduce time required for establishment of synchronization and makes sync detection with high precision possible.